THE DEEP CRUSTAL ROOT OF AN EXTENSIONAL SHEAR ZONE: CAPRICORN RIDGE, MT. HAY BLOCK, CENTRAL AUSTRALIA

The Mt. Hay block is a ~12-km-thick, deep continental crustal section exposed in the southern Arunta Region of central Australia. Mt. Hay granulites (~750°C, 8kb) deformed along a North Australian Craton continental margin during the 1740–1690 Ma Strangways event. The two main structural domains are the Mt. Hay ridge sheath fold (Bonamici et al.) and the cross-cutting, >6km thick, Capricorn ridge shear zone (CRSZ). After removing tilting primarily due to Devonian exhumation, the Mt. Hay fold restores to subhorizontal and the CRSZ restores to a 30- to 50°-dipping, normal-sense shear zone.

Parallelism of grain shape fabrics and lithologic boundaries and transposition of older fabrics indicate that the CRSZ accommodated high strain. Shape fabrics in volumetrically dominant compositional domains indicate oblate strain (as defined by felsic segregations and the orientation distributions of axes of anisotropy of magnetic susceptibility and remanence in gabbroic granulite, and plagioclase in anorthosite) to plane strain (as defined by pyroxene and biotite in gabbroic granulite). Oblate strain increasingly dominates across 100-m scale internal strain gradients, which center on the major lithologic boundaries. Similar measurements in the Mt. Hay sheath fold indicate dominantly plane to prolate strain (Bonamici et al.). Together, these observations indicate that deformation in the CRSZ combined shear and flattening.

The CRSZ therefore represents relatively localized extensional deformation in the deep crust (26–30 km). There is no evidence of reactivation as the CRSZ cross-cuts the earlier foliation by 30-50°. The low temperature difference between the CRSZ and its footwall (<60°C using two-pyroxene geothermometry), despite high shear strains across its >6km thickness, possibly indicates that the CRSZ was a relatively low-angle normal shear zone. Extension reported for the later stages of the Strangways event did not result in widespread continental rifting, although interactions between Archean and Paleoproterozoic crustal components assembling to form Mesoproterozoic Australia could have resulted in regional extension. Rather, the Mt. Hay block likely records the transition from convergence along a magmatic arc to (local) extension, such as a back-arc setting, during the Strangways event.